This invention relates to quadrature couplers, and particularly, to such couplers in the form of a spiral having at least a portion of the outer conductor with a wider width than inner conductors or interdigitation of portions of the spiral lengths.
As the size of microwave integrated circuits have become substantially reduced in recent years, particularly in the microwave industry in which monolithic integrated circuits are becoming increasingly common, a small-sized quadrature coupler is needed. Early developments included an interdigitated strip line coupler and variations of it, such as are described in U.S. Pat. Nos. 3,516,024, issued to Lange; 4,636,754, issued to Presser et al.; and in Kemp et al., "Ultra-Wide Band Quadrature Coupler", Electronics Letters, vol. 19, no. 6, pp. 197-199, Mar. 17, 1983. These couplers are a quarter-wavelength long for a single section and require additional space for multisection couplers.
In order to obtain a coupler of even further reduced size, a spiral coupler was developed by Schibata et al., as described in "Microstrip Spiral Directional Coupler", IEEE Transactions on Microwave Theory and Techniques, vol. MTT-29, no. 7, July 1981, pp. 680-689. This coupler is formed by coiling two edge-coupled conductors either as two continuous conductors coiled in 11/2 or 2 turns, or by connected conductor portions which are coiled and connected by jumper wires to form an equivalent number of turns. These couplers use conductors which are all of identical width and require a gap length of 3/4 of a wavelength of a selected design frequency to achieve a 3 dB level of coupling. However, because of the coil configuration they are able to achieve a reduced size as compared to straight strip line couplers. However, as suggested by Shibata et al., the couplers are only good up to a limited frequency, such as three GHz.
The present invention overcomes the limitations of these known devices. For instance, the invention provides a spiral coupler which has a gap length approximately equal to the gap length of the straight strip line couplers, but is substantially smaller. The invention provides a coupler which provides for extended bandwidth over a higher frequency range than spiral couplers have heretofore provided. Further, the gap spacing of the coupler provided by the present invention does not have to be as narrow as is required in prior known spiral couplers for the same frequency range and performance.
More particularly, the present invention provides a spiral coupler made by forming a pair of coiled microstrip conductor lines on a substrate. The two spiral strip conductors are disposed generally in parallel, coplanar, and coextensive spirals. Four alternating coupled regions are formed comprising a first region and a third region in which the inner strip conductor and outer strip conductor are weakly coupled. A second region is disposed between the first and third regions in which the inner strip conductor and outer strip conductor are in the form of an interdigital arrangement of four strongly coupled conductor sections with adjacent sections connected to a different one of the inner and outer strip conductors. A fourth region is disposed between the third and first regions of strong coupling, wherein the beginning and ending portions of the coupled inner and outer strip conductors are adjacent each other as four generally parallel conductor ending portions. These parallel portions have widths less than the widths of the inner and outer strip conductors in the first and third regions.
Further, the present invention provides means for varying the bandwidth of the coupler after it has been constructed. In the preferred embodiment, this is comprised of conductors disposed as islands adjacent to the continuous spiral conductors. Jumper wires may be connected at spaced locations between one of the spiral conductors and one or more of the pads for varying the effective length of the conductors. These pads may be selectively coupled to the spiral conductors in order to obtain a desired bandwidth narrower than the design bandwidth of the coupler without use of such pads.
Such a coupler provides an extended and higher bandwidth at a reduced size as compared to previously known devices. These and other features and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiment.